III. B. Coastal Processes |
As illustrated in Figure II-1, the zones in which coastal processes are active include the marine riparian zone, the backshore, the beach face (or normal breaker zone), and the low-tide terrace. Failure of the bluff face may from time to time change the landward position in this definition. Because waves in Puget Sound are generally small, the point at which waves would not affect sediment movement would occur well seaward of the lower limit of the photic zone. Natural forces and human influence constantly mold the coastal environment.
The beach component of the nearshore environment is defined as the profile of the shore in which sediment is moved by wave forces. This area includes the backshore to the limit of high water, the beach face, the low-tide terrace, and an offshore zone (Figure III-6). Though the bluff may contribute to the beach from time to time, it is not part of the beach in this definition. The offshore zone is the seaward portion of the beach profile to a depth below which waves no longer affect the bottom sediment. The beach width is measured perpendicular to the shoreline, from the deepest depth where the most extreme waves cease to cause sediment movement to the landward limit of wave run-up (Komar 1998a).
Figure III-4a. Spit, marsh, & lagoon at Battle Point. (© WA Dept of Ecology 2001).
Figure III-4b. Cuspate foreland at Yeomalt Point. (© WA Dept of Ecology 2001).
Figure III-5. Rock outcrop at Restoration Point (© WA Dept of Ecology 2001).
Figure III-6. Tidal datums for the Seattle Station (Source: NOAA 2001)
The Bainbridge Island beaches are generally characterized as having steeply sloping, gravel and cobble beach faces, narrow to no backshore widths, and extensive low-tide terraces. This type of beach seldom possesses a true surf zone as may be found on ocean beaches; instead, the waves break close to shore and develop directly into an intense swash that runs up and then back down the beach face (Komar 1998a). The breaking waves are effective in mobilizing the fine sediment, which is transported in suspension to be deposited on the low-tide terrace or in deeper water. The coarse sand and gravel that remains behind as a lag deposit forms an effective veneer that protects the high-tide beach and is moved by only the largest waves. There are some protected inlets, bay areas, and cuspate forelands where the beach slope is more gradual, with a beach face composed of gravel and sand.
This section addresses the nearshore physical processes that occur in Puget Sound, focusing primarily on those important to Bainbridge Island. Tides and the changing sea level are presented first, followed by effects of waves and currents. Finally, we address sediment transport and deposition processes.
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